Scanning current generator



Patented Sept. 10, 1940 UNiTED STATES PATENT I orrics SCANNING CURRENTGENERATOR corporation of Delaware Application February 10, 1936, SerialNo. 63,078

5 Claims.

This invention relates to generators for producing current waves ofsaw-tooth form incircuits containing both resistance and inductance,particularly as such circuits are used for the generation of scanningfields in oscillight or cathode ray oscillograph tubes as used fortelevision reception.

In my prior copending application, Serial No. 449,984, filed May 5,1930, entitled "Television scanning and synchronizing system, I haveshown that inorder to produce such waves by means of thermionicamplifying tubes the control voltage applied to, the grids of such tubesshould be an extremely powerful pulse in one direction, so applied as tocause the sudden change of current from maximum to minimum (or frompositive maximum to negative maximum) followed by a gradual and uniformchange of voltage to produce a gradual straight-line rise of currentfrom minimum to maximum. This wave form is necessary because the voltagenecessary to overcome the inductive reactance of the circuit is directlyproportional to the rate mum the inductive component of the voltage isby far the larger, and hence the necessity for the powerful pulse.During the remainder of the cycle, where the rate of change of currentis preferablynot over two or three per cent-as great as during theperiodof decreasing current, the inductive component of the voltage may bepractically negligible, and the resistance component becomes dominant.

In the earlier copending application above referred to, a current waveof the desired form was passed through a distorting network-in order toproduce the desired voltage wave shape. The present invention relates toa method and means for using a succession of relatively short, widelyspaced current pulses to produce the required wave form, these pulsesbeing easily generated locally or derived from a received synchronizingp e Among the objects of this invention are: to provide a means andmethod of generating sawtooth waves of true straight-line form in an inoduct-ive-resistive circuit; to provide a means of generating such waveswherein the fly-back time is a very small part of the entire cycle; toprovide a means of converting an oscillation consisting of short widelyseparated current pulses into a saw-tooth wave; to provide 'a'means "ofgenerating a voltage pulse of the necessary amplitude to overcome theinductive reactance as required to secure an exceedingly short fly-backtime; to provide a, means of utilizing received or locally generatedsynchronizing pulses directly to produce saw-tooth waves, without theintervention of an electronic oscillator; and. to provide a means ofcorrecting the exponentially curved wave form which is derived from thedischarge of a condenser to produce a true straightline wave form. l

My invention possesses numerous other objects and features of advantage,some of which, together with the foregoing, will be set forth in thefollowing description of specific apparatus embodyingand utilizing mynovel method. It is therefore to be understood that my method isapplicable to otherapparatus, and that I do not limit myself, in anyway, to the apparatus of the present application, as I may adopt variousother apparatus embodiments, utilizing the method, within the scope ofthe appended claims.

Considered broadly, this invention comprises suitable means forproducing widely spaced short current pulses of the type abovedescribed,and a wave-forming network through which these pulses are passed. .Thisnetwork includes at least one parallel resistance-capacitance circuitwhich is connected in series with a simple resistor and a suitablesource of current and voltage. In its limiting form such apulse-generating oscillator will alternate between the condition whereinit forms an element of practically zero resistance in series with thewave-forming circuit just described and the current source, and a secondcondition wherein it forms an open circuit,

resistor which may be utilized as the pulse required to overcome theinductive reactants and current, flowing through .the resistor in theresistance-capacitance circuit, imposes a charge upon the condenser inparallel therewith. when the second condition occurs all flow of currentin the resistor ceases, and the only voltage remaining in thewave-forming circuit is that due to the charge upon the condenser. Thedischarge of this condenser 'causes the gradual .55

- cause the necessary rapid fly-back. This same change of currentdesired for the second portion of the control-voltage cycle. Thevoltages thus produced are applied through a suitable network to controlan amplifier having a nonlinear characteristic, and these voltages areso applied that, considering equal small increments of time at thebeginning and end of the cycle of condenser discharge, the relativelysmall changes of potential at the end of such cycle produce the samevariations in current in the output of the amplifier as do therelatively large changes at the beginning of the cycle of condenserdischarge.

It is to be understood that in speaking of the cycle of condenser chargeand discharge, what' is referred to is the degree of charge anddischarge which are actually achieved by such condenser in the operationof the device, and that this cycle is relatively short compared with thetime required for complete discharge of the condenser.

Referring to the drawings:

Figure l is a circuit diagram illustrating my invention as actuated by akipp-tube oscillator.

Figure 2 is a series of curves showing the voltages and currentsappearing in various portions of the circuit of Figure 1.

Figure 3 is a diagram of a modified form of circuit embodying theinvention.

Figure 4 is a diagram of the circuit of the final stage of a televisionamplifier, showing the connection of the oscillight tube and the highandlow-frequency scanning-current generators.

Figure 5 is a diagram of the invention as applied to the high-frequency.scanning circuit of a television receiver.

The operation of the system here disclosed can better be understood byreference to a preferred form of the device as shown in Figure 1. Inthis figure only the essential'circuits are shown, the

' conventional filament-supply circuits being omitbe that type ofthermionic tube marketed under the designation of Type 56, and in thedescription which follows circuit constants will be given which aresuitable for the generation of sawtooth waves of fromtwenty to sixtycycles per second with a tube of this character, but it is to beunderstood that these constants are illustrative only, and that byvarying them suitably both widely different frequencies may be generatedand widely diiferenttypes of tubes may be used, and that the changesnecessary 'to accomplish this may readily be deduced from the valuesgiven by the use of well-known engineering principles.

' The cathode 2 of the tube l is shown as grounded. The grid 3 isconnected through the secondary winding 5 of a small transformer to atiming circuit comprising a condenser E, which may have a capacitance ofthe order of 0.25 microfarad, in parallel with a variable resistor 1whose maximum resistance is conveniently of the order of-250,000 ohms,and thence to ground.

The plate or anode 8 of the tube l connects through the primary windingIU of the above-v mentioned transformer to the wave-forming circuit,which includes the pulse resistor l I and an integrating circuit I 2comprising a resistor l3 of the order of 200,000 ohms in parallel withadirectly in series with the usual plate-current supply l6 and thus toground, but I prefer to include The operation of the circuit as thus fardescribed is as follows: When the device is put into operation there isno bias upon the grid 3 and current passes in the plate circuit,including the transformer primary winding I0. This induces a voltage inthe secondary winding 5 of this transformer (which preferably has astep-up turns ratio of about 1.5 to l), the transformer being so poledthat increase of current in the primary winding swings the gridpositive. This not only causes an increased flow in the plate circuitbut also causes current flow to the grid, until charging of thecondenser 6 and the collection of the negative charge upon the gridcauses the current to start to diminish. This at once tends to swing thegrid negative, due to the action of the transformer, and causes acomplete blocking of the tube which persists until the charge leaks offof the condenser 6 through the resistor l, whereupon the cycle repeats.ii'he curves 20 and 2| of Fig. 2 show the approximate waveform-of theplate and grid currents, respectively. V

The'time required for the charge to leak off of the condenser 6 iscontrolled by varying the resistor l, and is, in practice, much longerthan would be indicated by the curves, the ratio of the length of thecurrent pulses to the interval between such pulses being in practicefrom to A The voltage on the grid 3 of the tube I is shown in likemanner by the curve 22 of Figure 2.

The current I as shown in curve 20, flowing in the integrating circuitl2, causes a voltage drop across the condenser l5 as shown by thefalling portion of curve 23, and onto this voltage drop is superposedthe drop caused by the flow in the resistor ll. during the period offlow which is shown by that portion of curve 25 between the origin 0 andthe ordinate A. At this point the current suddenly ceases to flow, andcurve 23 illustrates the This gives a voltage rise of voltage during theperiod of discharge end 01' the wave-forming network and ground isapplied across the input or control circuit of an amplifier through anetwork which comprises a blockingv condenser 30 having a capacitance ofabout 1.0 microfarad, in series with two resistors 3i and 32 of a valueof about 250,000 ohms each, which form a voltage divider.

The output amplifier, which, is connected across the resistor 32,comprises in the present instance two more type 56 tubes, 35 and 36,whose plates 31 and 3B are connected in push-pull relationship across adeflecting magnet 40, which is illustrated merely as. an iron-core coil.The center tap of this magnet connects through the lead 4| to the supplysource l6.

' The grid 42 of the tube 35 is connected to the junction of theresistors 3| and 32, and the filament or cathode 43 of this tubeconnects to ground through a resistor 45 of about 1000 ohms. Thefilament 46 of tube 36 is also connected to ground through thisresistor, whereas the grid 41 connects directly to ground.

As a result ofthis asymmetrical connection the two tubes are unequallybiased and loaded. The major portion of the control voltage is appliedacross tube 35, whose grid swings positive during a part of the cycleand therefore acquires a bias through the resistor 32. Additional biasis supplied to both tubes through the resistor 45. The unbalancedconnection of the tubes and the unequal application of voltage to themresults in a curvature of the grid-plate characteristic of the two tubesconsidered as a unit, and this is .in opposition to the curvature of thedischarge characteristic of the condenser l5. It is difiicult, however,to get an exact balance in this manner, and the final correction may beapplied by connecting two similar small condensers 49 and 50 across theresistors 3| and 32. The proper size for these condensers can speedilybe found" complex as to obscure what actuallytakes place,

but it is possible to break down the circuit into its component elementsand so to obtain an approximate picture of what happens. It should benoted first that the frequency of the oscillation is dependent almostentirely on the relationship between the condenser E and the resistor I,and

that the efiect of the coupling circuit upon the oscillation period maybe neglected. It should also be noted that the circuit is quitedifferent during the current pulses and in the interval between thepulses.

The pulse period itself is so short in comparison with the time constantof the various circuit elements that the condensers l5, l8and 30 can.

be entirely neglected and thought of as replaced by direct metallicconnection. Condensers 59 and 50 in series may therefore be consideredas forming one arm of a parallel circuit whose other two arms arerespectivelythe pulse resistor II and the resistors 3| and 32 in series.Because of the relatively high resistance of the elements 3| and 32,they too may be neglected and the pulse may be considered as suppliedthrough the resistor (2,000 to 5,000 ohms) in parallel with thecapacitance of 0.025'microfarad. The time constant of such a combinationis of the order of 5x 10 which is so small that even in theshortestfly-back time contemplated in this arrange-- ment, thepulse voltage isreduced by much less than one per cent by the'presence of thesecondensers.

When the tube I becomes non-conductive the dominant elements in thenetwork are the integ'rating circuit I2, which has a time constant ofapproximatelyOl and a branch comprising grating circuit that thepresence of the condenser 30=may, as a first approximation, be neglectedtoether with the resistance Illand the network the tube I biased tocutoff, through the lead 62.

the cycle when the tube is non-conducting are those which may becomputed by considering that a short-circuit across'this latter branchof the circuit has suddenly beenfremoved. The solution of this circuitshows it to be in the form 1Ice- The presence of the small condensers 49and 50 decreases both It and a; that is, it reduces slightly theamplitude of the voltage variations as applied to the amplifier andincreases the time constant of the circuit, straightening the curve. Thedecrease in amplitude is relatively slight for small values ofcapacitance, but the increase in time constant is of sumcient magnitudeto be of material value when applied as a final correction. In thesecond approximation,

wherein the effect of the series condenser 30 is considered, thecomplete solution is very complex, but the over-all effect is again aneffective increase in time constant, straightening the curve stillfurther with a slight decrease in amplitude.

The final curve of voltage rise across the amplifier may be made asstraight as desired, but it is preferable practically tolleave somecurvature in the applied voltage curve and to apply a small but definitepositive correction by means of the amplifier characteristic as abovedescribed.-

With the constants above given the departure from linearity thus to becorrected is less than 1.5 per cent at a frequency of 24 cycles, andisentirely negligible at a frequency of 60 cycles.

The saw-tooth oscillator thus described is selfos'cillatory withconsiderable stability, but-it may readily be synchronized by injectinga small voltage of the desired frequency into the transformer whichcouples the grid and the plate circuit of This may be done by anadditional small winding 5| coupled to the transformer core and fed fromany desired source.

Figure 3 shows a form of the invention which is primarily adapted foruse with television transmitters where it is desirable to have thefrequency absolutely controlled by external mechanical means. The short,widely separated positive pulses are fed to a tube 6|, which is normallyThe output circuit of the tube 6| is similar to that of the embodimentfirst described, and the circuit elements are designated by the samereference characters distinguished by primes. It is assumed, however,that the tubes and 36' are tubes of low amplification factor and highcurrent-carrying capacity. In order to obtain the necessaryamplification the tube 35 is therefore used to drive-the tube 36'through a circuit comprising a large blocking condenser 63 in serieswith a resistor 65, which connects to the grid 41' and to ground througha resistor 66. This connection gives greatly increased amplification,particularly of the-pulse, and the pulse resistor H can therefore bereduced in size. The

biases on the tubes 35' and 36' will usually have to be somewhatdifferent from those shown in the embodiment of Figure 1 in order to getthe desired corrective asymmetry of the grid-plate characteristic of theamplifier, but this is de- I cathode 92 is self-biased almost to cutoffby a,

to be utilized directly to efiect scanning without the intervention ofany kipp tube or local oscillator whatsoever.

As has been shown in my copending application Ser. No. 449,984, and asis now the universal practice in electronic television systems, a pulsesent out between each two lines of scansion, and another transmittedbetween each two picture frames, may be utilized for the dual purpose ofholding the receiver in step and of eliminating the back lines in thereceived picture. Figure 4 shows the method of applying the presentinvention to accomplish this directly. The combined pulses and picturesignals, amplified to the required degree by any convenient type ofampli-' fier I0, are fed through a blocking'condenser H to 'the controlgrid of a final power tube I2 which further amplifies the signals andapplies them through a blocking condenser I4 to the grid of theoscillight I5, the output of the amplifier I being so phased that thesynchronizing pulses comprise positive swings as applied to the grid ofthe power tube, by which they are reversed in phase and applied asnegative swings to the oscillight, erasing the back lines.

The tube I2 is provided with the usual grid leak I6. Also connected tothe grid is a resistor 11 to which the high-frequency and low-frequencyscanning units are connected through a pair of conjugate filters, thehigh-frequency unit connecting to the resistor 'I'I through a smallcondenser I9, with a resistor 80 as a shunt element across the scanninggenerator input, while the low-frequency unit is preferably connectedthrough a series resistor 8I with the condenser 82 as the shunt elementThe output of the highfrequency unit 83 connects to a set ofhighfrequency scanning coils 84, while the output of the low-frequencyscanning unit 85 connects to the low-frequency deflecting coil 86. Thecircuits of the scanning units 83 and 85 differ essentially only intheir circuit constants, and hence only the high-frequency unit 83, asshown more completely in Figure 5, will be described in detail.

Referring to Fig. 5, the positive synchronizing pulse which appearsacross the shunt resistor 00 is applied to the grid 90 of a triode 9|;whose resistor 94, a large by-pass condenser 95 being shunted acrossthis resistor to maintain the cathode at ground potential insofar asalternating-current components are concerned. The plate of the tube 9|is connected through a pulse resistor 96 and integrating circuit 01,operating as has already been described, to a source of plate potentialindicated by B+. The positive pulses render the tube 9| instantaneouslyconducting, and the pulses charge the integrating circuit 91, as hasalready been described, to develop the desired wave form. For thisparticular use, however, where light weight and simplicity are paramountfactors, the simplest type of network is used to couple the succeedingamplifier, in this case simply a blocking condenser 99 and grid resistorI00, of the order of magnitude of 2.0 mic-- rofarads and 100,000 ohms,respectively, transferring the saw-tooth wave to a heater-type tube IOI.Thisstube is parallel-fed through a plate reactor I02, its outputcircuit being through another blocking condenser I04, also preferablyhaving a capacitance of about 2.0 microfarads, and a variable resistorI05, having a maximum resistance of about 200 ohms, to the twohighfrequency scanning coils 84 whose combined inductance is about 2.6millihenries.

In this case the wave form applied to the tube IOI is not as highlycorrected as in the embodiments previously described, but since a singleamplifier tube is used its plate characteristic will have somewhatgreater curvature, so that satisfactory linearity of wave form may beobtained. The final adjustment of this linearity is made by varying theresistance of the element I05.

This same type of high-frequency scanning circuit has also been used fortransmitters, but whereas for receiver scanning units a single amplifiertube of the 2A3 type, for example, may be used, for the transmittersseveral of these tubes will be used in parallel.

Where this type of circuit is used for the lowfrequency scanning, thecontants of the pulse resistor and integrating circuit will be of aboutthe same magnitude as those of the first embodiment described, insteadof using an integrating circuit with a time constant of about 0.0005 asin the high-frequency unit. The blocking condenser 99 will have the samevalue as in the high-frequency unit, but the grid leak I00 will be twoto three times as great in resistance. The

plate reactor I02 and blocking condenser I04 will.

also be much larger, and I have also found it more convenient to use atype 2A5 output tube than the 2A3 which has proved most satisfactorythus far in the high-frequency units. All of these modifications forchange in frequency are, however, exactly what would be expected fromordinary engineering considerations, and many other variants .of thesystem will naturally suggest themselves to designers in the art.Furthermore, it will be apparent that where the oscillight issufliciently sensitive to control voltages the phase-reversing amplifiermay be ,used in cascade with the scanning generators instead of with theoscillight, the primary requirement being merely that the pulses beapplied to the oscillight and generators in opposite phase.

'I claim:

1. The method of producing synchronized sawtooth scanning waves inreactive scanning circuits in a receiver for television signals whichinclude pulses adapted to extinguish return lines in the receivedpicture, which comprises the steps of amplifying said pulses withreversal of phase so that said pulses appear with opposite signs indifferent portions of said receiver, ap-

plying said pulses in one phase to cause current flow in one portion ofsaid receiver, producing a distorted voltage wave from said current flowof proper form to produce a saw-tooth wave in a circuit containingreactance, applying such saw-tooth wave to scan a picture field,'andapplying said pulses simultaneously in opposite phase to suppress flowin another portion of said .receiver.

2. The method of producing synchronized saw-tooth scanning waves in"reactive scanning circuits in a receiver for television signals which.include pulses adapted to extinguish returnlines in the receivedpicture, which comprises the steps of amplifying said pulses withreversal of phase so that said pulses appear with opposite signs indifierent portions of said receiver, applying said pulses in positivephase to cause current flow, distorting and amplifying said current flowto produce a scanning wave, and simultaneously applying said-pulses innegative phase to suppress a current flow to extinguish the return linesin the received picture.

3. The method of producing synchronized saw-tooth scanning currents inan inductive and resistive scanning circuit in a receiver for televisionsignals which include pulses adapted to extinguish return lines in thereceived picture, which comprises the steps of amplifying said pulseswith reversal of phase so that'the pulse appears in diflerent portionsof said receiver with opposite signs, applying said pulse in positivephase to cause a current fiow in said scanning circuit substantiallycoincident therewith in duration, applying a potential in phase withsaid pulse to cause a sudden change of said current flow, limiting thesubsequent changesof current in said scanning circuit to a substantiallyconstant rate until againchanged by a pulse, and applying said pulses innegative phase to extinguish the return lines simultaneously with saidsudden change.

4. A receiver for television signals which include pulses adapted toextinguish return lines in the received picture, comprising means foramplifying said pulses with reversal of phase so that said pulses appearwith opposite signs in different portionsof said receiver, means forapplying said pulses in one phase to cause current flow in one 'portionof said receiver, means for producing a distorted voltage wave from saidcurrent flow of proper form to produce a saw-tooth wave in a circuitcontaining reactance; means for utilizing such saw-tooth wave to scan apicture field, and means for applying said pulses simultaneously inopposite phase to suppress flow in another portion of said receiver.

5. A television signal receiving. system comi prising scanning circuitsadapted to be actuated plifier for distorting the amplified pulses todevelop therefrom a saw-tooth scanning wave, and means for utilizingsaid scanning wavelto actuate said scanning circuits.

PHD T. FARNSWORTH.

